The modern Roman Catholic Church has been shaped by two men: Angelo Roncalli, Pope John XXIII, and Karol Wojtyla, Pope John Paul II. The enormous changes that have swept Catholicism over the past 36 years cannot be understood without grasping the characters, beliefs and work of these two men -- both great Popes but very different Popes. John XXIII, TIME's 1962 Man of the Year, was nearly 77 when he came to the throne of St. Peter, and his reign lasted less than five years, from 1958 to 1963. John Paul II was by papal standards a comparatively young man when...